Biased series-type noise limiter and silencer



May 12, 1953 E. TOTH BIASED SERIES-TYPE NOISE LIMITER AND SILENCER.

Filed Oct. 31, 1947 I4 AUDIO OUTPUT AUDIO OUTPUT .ooo|

IMEG.

EMERICK TOTH Patented May 12, 1953 n gsspslin Es-IrYP NQEELIMIIE R Emerick 'lloth, Takoma. Bark, erriicaiicnflciptsr 3. die-stal ing. 'i tttl.

1. Claim. (01. 250-20), Gr ntedwlcer. T t e 35, U. a. Co e. (Lt-tat,

'llhis invention relates. generally ted-10G 379 noise. limiting. circuits for radio. receiyers, more particularly to a. circuit for.- opposing the. internal: potential of; the noise. limiting diode.

A study of diode characteristics reveals that the thermal emission of the heated. cathodeproduces an equivalent internal. potential oft about 1.2;- volts. between cathode. and. plate. in: diode such. as the. type. 6H6; independent of. any exter: nal-ly. applied potential. Obviously a. diodefs in-, ternal potential will bemoreeftective it-the exter-. nal potential is small but will. be. negligible in. comparison to a large external potential. Accordingly, the. influence. oi the. internat potential: varies with the relativemagnitude of the, extere nal' potential and hence the. diode. characteristies are. non-linear except where the in-ternat potential is negligible. with respect to. the externat potential. Similarly, the. internal resistance of a diode varies with the applied: voltage.

Conventional noise limiting. circuits of: the. se:

ries diode type generally-provide a delaybiaslor the limiter diode which. is. controlled: by. the aye1'-.. age. amplitude of the incoming signal. Such circuits are usually designed to. limit signal peaks. exceeding a certain percentage of modulation, With this arrangement the circuit maybe expect: ed to limit at thedesired level of modulation peraser espe ti e c he av e Signal ampl tude. Bowel 6 in practice it is found that if the average signal is not large with respect to the limiter diodes internal potential, the limiter diode wiil have. its threshold at a much higher level of modulation percentage than that for which it designed. This is true since a signalpeak must overcome. the diodeis internal pQten-i.

tial as well its delay. bias order to operate.

the. limiter. If the. average signal is large, the delay bias is large. and the diode potential is negligi ble with respectthereto. However if theaverage signal is Slim/11313 128 delay bias may be equal to or less than the diode potential and a signal peak may reach twice or more the desired level of modulation before it will operate the limiter. The limiter circuit will thereby be less effective just when it could be most useful, i. e., in the reception of weak signals.

It is therefore an object of this invention to provide a noise limiting circuit which permits operation of a limiter diode at the same predetermined level of modulation percentage durin the reception of either strong or weak signals.

It is another object of this invention to provide a circuit for rendering ineffective the internal potential of a diode type vacuum tube.

ether piece and a reatcses '1- the nve t on. ll: be cream... frj m e olloa na des t d scms n ins dra ngs. wherein.- hara t rs of; re erence nd cate m ar n tile -s una Fig. l is a schematic diagram of; a preferred mbodiments?-thisi riren endis c em dia ram Qt alternatement ot-t smrea o to". his invent on usclc e a er es iode yne. noise. m ti c i whe e n; source. t al: s apnli d o; h oise lim ter d ode s r ee t ts de a ed ias vol age- Sa d. z-Potentia r an o o ar ty asito oppose. u r t. thr u the. imiter.- d odehe. see. of:- potent al; m y be ncreased: masnie.

' .c a a. squelchor i cnc rcircu tto. n d t i a. c ema ic an c am a tai o the. demodu at on. sect on of: a radio receiuer, The demodulatorissupplied with h signs-tr atis throu h E: tran fao mert. h ene s dete ed: by a, detector diode. 6i and the etecte i a s ead-oped across. a. pair.- ofr.e-. sisters an t. se ies. T e R;- F. si nal: com:

lite

- potent bynasse around resistors 1; and; a

through a c pacitance The plate ci a. imiter diode, I10: is tied o. the junction of resistors. l.- and: 8 and. thetotal recti... fled signal voltage developed across. said resistors supplies. the, delay bias tor the cathode; of said diode. through resistances. H- and. I2 whose junc-. tion is bypassed to. ground; by a condenser. t3. As, indicated in the drawing, the respective. value of components ti 52 and It is such that the voltage. at their common terminal will not follow audio. frequency variations of the signal voltage. The audio. signal output' it is taken from the cathode of diode Ill across resistance tz and capaci-tance t3. H W

the internal resistance and potential or limiter diode it is schematically shown in the drawa resistor i5 a' battery. H5 in series with the plate and cathode. It should be noted that the positive side of the battery It is shown nearest the cathode. Using the conventional notation for the direction of current flow, the battery It? will produce an extenral current from the cathode to the plate of diode l6 through resistances 52, H and 8. This current is indicated in the drawings by a series of arrows marked B.

A relatively low resistance potentiometer I7 is in series with resistance [2 and the junction of resistance H and capacitance I3. The series connection is arranged so that resistance 12 connects to the tap it of the potentiometer and resistance H connects to one end thereof. A battery E3 is in parallel with potentiometer I! such that its negative terminal is at the junction of resistance H and capacitance iii. In the fore going discussion elements ll, i8 and I9 have not been considered, it being assumed that these elements were eliminated from the circuit by adjusting tap Hi to the left end of potentiometer IT.

A potential which is equal and opposite to the internal potential of diode l may be introduced to the cathode circuit of said diode by moving potentiometer tap [8 to the right. The potential thus introduced will produce a current flowing from tap it through resistance I2, diode I0, resistances 8 and H and back to potentiometer $1. This current is indicated by a series of arrows marked A in the drawing. It will be seen that when the potential introduced by potentiometer I1 is equal and opposite to the internal potential of diode 50, their respective currents A and B will be equal and opposite and will effectively cancel each other. Thus, under these conditions the internal potential of diode I0 is balanced out within the circuit and will not have to be overcome by the detected signal before the limiting action may occur.

The detector diode 6 will also produce an internal potential, and although it is of less consequence, it produces a current through resistances l and 8 and through resistances 1, l2, and H and diode Ill. The currents as produced are designated by a series of arrows marked C in the drawing. The current produced in resistance 8 is most important for here it effects the delay bias of the limiter diode. By properly setting tap IE on potentiometer H, the current A can be given just the proper amplitude to cancel currents B and C, thus a proper setting of tap I8 overcomes the internal potential of both the detector diode 6 and the limiter diode 10.

In Fig. 2 an alernative means for providing a bucking bias for the series limiter diode circuit of Fig. 1 is shown. Elements l1, I8 and I9 are omitted from Fig. 2 and resistance I2 is connected directly to the junction of resistor II and capacitance !3. To this junction is also connected high resistance 20 which couples the j unction to a tap 2| on a potentiometer 22. The terminals of potentiometer 22 connect between a positive voltage source 23 and the common ground 24. By a suitable adjustment of tap 2!, a bucking voltage equal and opposite to the diodes internal potential may be introduced to the circuit at the junction of resistors I2 and II. The bucking voltage is coupled through the high resistance 20 in order to prevent potentiometer 22 from shunting capacitance l3 with a low impedance path.

The respective potentiometers of Figs. 1 and 2 may also be used to provide squelching or silencing action in the limiter circuit. By setting the taps of these potentiometers so that a substantial positive voltage is introduced to the cathode of .4 diode 16, said diode will remain non-conductive until a sufiiciently larger potential appears at the plate. With this arrangement the taps l8 and 2| respectively may be set so that diode 10 will not pass random background noise to output 14 in the absence of a detected signal. When a detected signal appears across resistances and 8, a negative delay bias is introduced to the limiter cathode through resistances II and 12. If the detected signal is sufiiciently large, the delay bias will overcome the positive bias manually applied to the cathode and the signal voltage will appear at output I4.

Although only limited and specific embodiments of the present invention have been shown and described, it may be modified without de parting from the principles thereof and any such modifications are, therefore, deemed to be within the scope of the invention as defined in the appended claim.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes Without the payment of any royalties thereon or therefor.

What is claimed is:

In a radio receiver, a demodulator comprising, detecting means including a first diode and a first impedance connected in series, an output terminal, a second diode serially connecting said output terminal to an intermediate point in said first impedance, an output load impedance having one end connected to said output terminal, a resistance-capacitance time constant circuit having its resistance leg serially connecting the other end of said load impedance to one end of said first impedance to supply the average level of the detected voltage to said second diode as a delay bias, a voltage source of higher potential than the combined contact potentials of said first and second diodes, a voltage divider connected between said voltage source and the other end of said first impedance, said voltage divider having a variable tap, and high impedance means connecting said variable tap to the junction of the resistance leg of said time constant circuit and said load impedance.

EMERICK TOTH.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,966,047 Ryall July 10, 1934 1,990,782 Greiback Feb. 12, 1935 2,039,267 Barber Apr. 28, 1936 2,248,267 Bacon July 8, 1941 2,301,620 Fowler Nov. '10, 1942 2,518,449 Coe Aug. 15, 1950 OTHER REFERENCES Cooking: The Diode Detector, Wireless World for February 3, 1938, pages 103 to 105. 

